A major consideration in the design of transmitters for wireless devices is the efficiency of the power amplifiers and related circuitry. High efficiency is especially important in portable devices, as transmitter power consumption is typically a major determinant of a device's battery life between charges. Recent trends in mobile wireless communication systems are placing higher demands on power amplifier performance. In particular, data rates supported by wireless systems continue to climb; these higher data rates are enabled by more complex modulation schemes and tighter control of interference from wireless transmitters. These complex modulation schemes and interference limitations make high operating efficiency more difficult to achieve.
Conventional power amplifiers, such as class-B amplifiers, perform with maximum efficiency only at power levels at or near maximum saturated output power levels. However, operation at these levels generally causes high nonlinear distortion, intermodulation products, and harmonics. Nonlinear power amplifier performance is particularly problematic for radio signals with an amplitude modulation; for this reason many early wireless systems were based on constant-envelope modulation schemes such as Frequency Modulation (FM), Frequency-Shift keying (FSK), and Gaussian Minimum Shift Keying (GMSK). However, today's high data-rate systems employ more complex modulation schemes (e.g., the Single-Carrier Frequency Division Multiple Access scheme specified by the 3rd-Generation Partnership Project, or 3GPP, in their Long-Term Evolution initiative), where information is conveyed in both amplitude and phase/frequency components of the modulated signal.
Transmitter circuits may generally employ either a linear architecture or a polar architecture. In a linear-architecture transmitter circuit, a radio frequency signal modulated in both amplitude and phase is amplified directly. Signal distortion is generally reduced to acceptable levels by operating at output signal levels significantly below the saturated output power level, although various techniques such as signal pre-distortion may also be employed to improve the linearity of the amplifier circuit.
Polar transmitter architectures offer the potential of higher efficiencies. In a polar transmitter, the phase modulation component of the signal to be amplified is separated from the amplitude component. Various configurations of polar transmitter circuits have been developed including the Envelope Elimination and Restoration (EER) and Envelope Tracking (ET) amplifier circuits, Pulse-width Modulation (PWM) amplifier circuits, and amplifier circuits employing so-called Linear Amplification with Non-linear Components, or LINC. Each of these systems relies on a transformation of an input information signal from a Cartesian coordinate representation (e.g., a representation based on in-Phase, I, and quadrature, Q, components) to a polar representation (i.e., amplitude and phase). This approach allows the use of non-linear amplifier components operated at higher efficiencies over a range of output power levels—for example, amplifier elements operated in class-D mode may be used in PWM amplifiers.
However, the transformation of the information signal to a polar representation can result in a large expansion of the bandwidth required to process the amplitude and phase components of the signal. This bandwidth expansion is especially severe in cases where the modulating signal passes close to the origin of the I-Q plane, since the phase component of the signal then experiences a very rapid transition. The corresponding amplitude component in that region of the signal will have a narrow V-shaped notch. Capturing the transition accurately requires a wide bandwidth; if the bandwidth is limited, undesired signals are produced out of band, potentially exceeding the allowed amount of emissions. In some cases, the required bandwidth can be more than ten times that of the information signal to be transmitted. This increased bandwidth corresponds to increased power dissipation in the transmitter chain; in some cases the total transmitter efficiency might not be much better in a polar transmitter circuit than in a linear one.
This bandwidth expansion can cause several other problems. For instance, in an EER amplifier circuit, amplitude variation is typically handled by a DC-DC converter circuit that modulates the supply voltage of the power amplifier. This circuit must have a very wide bandwidth to avoid spurious emissions. However, the DC-DC converter must also have very small ripple and high efficiency, which are generally more difficult to achieve with a larger bandwidth. Similar problems arise in other polar transmitter architectures.
In the past, linear transmitter architectures have often been preferred for meeting stringent modulation accuracy and spurious emissions requirements. However, although linear transmitter architectures do not suffer from the bandwidth expansion issues discussed above, the efficiency of linear power amplifiers is limited, especially when the input signal level is backed off significantly from the circuit's maximum output power. This limited efficiency means that polar transmitters continue to be of major interest, in spite of the bandwidth expansion problems, since they can support the use of highly efficient switched mode power amplifiers. The bandwidth expansion problem, however, is especially problematic for signals with large modulation depth and wide bandwidth, such as those specified for LTE systems.